The present invention relates to electric power plants and more particularly to systems for controlling the blending of steam outputs from multiple steam generators in such plants.
In the operation of fossil fired power plants, plant availability can be significantly enhanced through the provision of one or more backup steam generators or boilers. For example, a plant may be provided in its original design or in a retrofit design with two boilers for one turbine or three boilers for two turbines. In addition to functioning as a backup replacement, a backup boiler may be operated with the primary boiler to supplement the generating capacity of the plant.
One of the primary causes of lengthy boiler shutdowns is the need to repair water leaks resulting from water tube corrosion. With increasing installation of coal fired plants as opposed to gas or oil fired plants, water corrosion becomes more problematical because of sulfuric acid, slagging and other corrosive effects of burning coal. Therefore, the need for steam blending plant designs and steam blending controls for such plants increases with increased installation of coal fired plants.
Power plants having provision for steam mixing have been disclosed previously. For example, a steam mixing system is disclosed in U.S. Pat. No. 4,060,990 issued to P. V. Guido, R. L. Criswell and A. J. Zipay on Dec. 6, 1977 and assigned to Foster Wheeler Energy Corporation one of the coassignees of the present application. In that steam mixing system, the steam outflows from two boilers are mixed in a header and then directed to a high pressure turbine, but no disclosure is provided on how the steam flows are blended during startup or shutdown of one boiler while the other boiler is fully operational.
The provision of system control for the blending of steam presents special problems especially during the startup of an off-line boiler and the transfer of steam flow demand to it. In particular, it is desirable that steam blending be achieved with stable plant operation, i.e. without upsets to steam pressure and temperature.